This invention relates to a shock absorber for use in such as a vehicle suspension system or the like and, more particularly, to a hydraulic shock absorber of the type comprising a dual cylindrical tube consisting of an inner tube defining a cylinder and an outer tube surrounding the inner tube and defining an annular reservoir chamber around the outer periphery of the inner tube, a piston slidably disposed in the inner cylinder and having a piston rod extending out of the dual tube, liquid being filled in the cylinder and in a portion of the reservoir chamber, and pressurized gas being filled in a portion of the reservoir chamber.
In operating shock absorbers of the aforementioned type, air will sometimes enter into the cylinder according to the so-called aeration phenomenon, wherein gas in the reservoir chamber is mixed with liquid and introduced into the cylinder and thereafter separates from the liquid during vigorous operating conditions, or according to pressure difference generated between the gas in the reservoir chamber and liquid in the cylinder on the upper side of the piston during downward movement of the piston. Since the operating characteristics of the shock absorber will be substantially impaired when gas enters into the cylinder, it is necessary to return the gas into the reservoir chamber.
Various devices have been proposed to return the gas entered in the cylinder to the reservoir chamber, for example, Japanese Utility Model Publication 48-25495 shows a lip portion formed integrally with a seal member which sealingly closes an opening of the dual tube through which the piston rod extends. The lip portion acts as a check valve allowing fluid flow from the interior of the cylinder to the reservoir chamber but preventing fluid flow from the reservoir chamber to the interior of the cylinder. The lip portion is formed to depend downward from a main body portion of the seal member, and thus, in manufacturing the seal member, it is difficult to reduce the thickness of the lip portion to a desired thickness, and therefore the sensitivity of the check valve is impaired. Further, since the lip portion and the seal member are formed integrally of one material, there are the shortcomings that when a material having some degree of rigidity is used to conform the requirements of the check valve the sealing characteristics against the piston rod will be impaired, when a soft material is used to ensure desired sealing characteristics the function of the check valve will be impaired. Further, the sliding movement of the piston rod relative to the seal member tends to carry the seal member itself in the same direction, and thus the function of the check valve will further be impaired.